The present invention relates to optical bodies containing cholesteric liquid crystals. The present invention also relates to reflective optical polarizers formed by simultaneous coating of two or more layers of cholesteric liquid crystals or cholesteric liquid crystal precursors.
Optical devices, such as polarizers and mirrors, are useful in a variety of applications including liquid crystal displays (LCD""s). Liquid crystal displays fall broadly into two categories: backlit (e.g., transmissive) displays, where light is provided from behind the display panel, and frontlit (e.g., reflective) displays, where light is provided from the front of the display (e.g., ambient light). These two display modes can be combined to form transflective displays that can be backlit, for example, under dim light conditions or read under bright ambient light.
Conventional backlit LCD displays typically use absorbing polarizers and can have less than 10% light transmission. Conventional reflective LCD displays are also based on absorbing polarizers and typically have less than 25% reflectivity. The low transmission or reflectance of these displays reduces display contrast and brightness and can require high power consumption.
Reflective polarizers have been developed for use in displays and other applications. Reflective polarizers preferentially transmit light of one polarization and preferentially reflect light having an orthogonal polarization. It is preferred that reflective polarizers transmit and reflect light without absorbing relatively large amounts of the light. Preferably, the reflective polarizer has less than 10% absorption for the transmission polarization. Most LCD""s operate over a broad range of wavelengths and, as a consequence, the reflective polarizer must typically operate over that broad wavelength range, as well.
In backlit displays, the reflective polarizer can be used to increase the efficiency of light utilization by reflecting the polarization of the light not transmitted by the polarizer back into the backlight. The backlight converts the polarization state of the recycled light for transmission through the reflective polarizer. This light recycling can increase total display brightness. In reflective and transflective displays, the reflective polarizer has lower absorptivity and color than most absorbing polarizers for the pass polarization of light, and can increase brightness of the display by up to 50% or more. Characteristics of reflective polarizers that are important to at least some applications include, for example, the thickness of the polarizer, the uniformity of reflection over a wavelength range, and the relative amount of light reflected over the wavelength range of interest.
Generally, the present invention relates to optical bodies containing cholesteric liquid crystals and their manufacture, as well as the use of cholesteric liquid crystals in optical devices, such as reflective polarizers. One embodiment is a method of making an optical body. A first coating composition and a second coating composition are coated onto a substrate with the first coating composition between the substrate and the second coating composition. The first and second coating compositions are different and include at least one cholesteric liquid crystal material selected from cholesteric liquid crystal compounds and cholesteric liquid crystal monomers. At least a portion of the second coating composition interdiffuses with a portion of the first coating composition. After interdiffusion, at least one cholesteric liquid crystal layer is formed from the first and second coating compositions. The coating of the first and second coating compositions can be done separately or simultaneously. Simultaneous coating methods include, for example, slide coating, curtain coating, and extrusion bar coating. Additional coating compositions (e.g., a third or fourth coating composition) can also be applied and, optionally, at least partially interdiffused with the preceding coating compositions. The optical body can be used as a reflective polarizer to form an optical display when combined, for example, with a display medium.
Another embodiment is another method of making an optical body. A first coating composition is disposed on a substrate and a second coating composition is disposed on the first coating composition. Each coating composition includes a solvent and a cholesteric liquid crystal material selected from cholesteric liquid crystal polymers and cholesteric liquid crystal monomers. At least a portion of the second coating composition is interdiffused with at least a portion of the first coating composition. After interdiffusion, substantially all of the solvent from both coating compositions, is eliminated. This can include, for example, evaporating a solvent or reacting the solvent to form a polymeric composition. At least one cholesteric liquid crystal layer is formed from the first and second coating compositions. The cholesteric liquid crystal layer can have a pitch that substantially continuously changes over a range of values along at least a portion of a thickness dimension of the layer.
Yet another embodiment is an optical body that includes a cholesteric liquid crystal layer. Along at least a portion of a thickness dimension of the cholesteric liquid crystal layer, a pitch of the layer begins at a first pitch, increases to a second pitch, decreases to a third pitch, and increases to a fourth pitch. This optical body can be used in an optical display as a reflective polarizer.
The above summary of the present invention is not intended to describe each disclosed embodiment or every implementation of the present invention. The Figures and the detailed description which follow more particularly exemplify these embodiments.